I bought a 340MB Microdrive two or two and a half years ago to go with my Canon S20 - it was a no-brainer then because of cost/MB compared to solid state cards. I haven't had any problems with it after about 6500 pictures.

I am getting an additional card for my new S40 for trips, and have been leaning toward a solid-state card, because:

no cost advantage to Microdrive at 340MB now (compared to 256MB solid state, in terms of cost/MB).

they use less power

moving parts will break down eventually. Although this is probably an irrational fear, the disk/heard doesn't move that often, and smaller parts are stronger (relatively), and since solid state also "wears out" after 100,000 cyles, there's probably not that much difference in reliability.